1. Field of the Invention
This invention relates to a photosensitive material to be used in the production of semiconductors, the production of printed circuit boards, and the working of a metallic article and more particularly to a photosensitive resin composition which is destined to form an etching resist useful for metalworking as in the production of a printed circuit board and the production of the shadow mask for a cathode ray tube. This invention further relates to a photosensitive resin composition useful for the formation of a varying sort of resin adhesive layer, the formation of a resist film such as an etching resist or a solder resist in a printed circuit board, and particularly the formation of a resin insulating layer containing via holes in the production of a multi-layer printed circuit board. This invention also relates to a method for the formation of a resist pattern or a resin insulating pattern using the photosensitive resin composition, and more particularly to a method for forming a positive resist pattern or resin insulating pattern by exposing to an active energy ray or actinic radiation a layer of the composition applied to a substrate thereby giving rise to a tack-free coating film, without inducing evaporation of an organic solvent, then subjecting the coating film to selective exposure to an active energy ray shorter in wavelength than the active energy ray mentioned above and to a heat treatment, and thereafter developing the exposed coating film with an aqueous alkaline solution.
2. Description of the Prior Art
The etching resists which are used for the formation of circuit patterns in the production of printed circuit boards or for the metalworking are classified under the pattern printing type and the developing type. As the etching resist of the pattern printing type, the composition which has varnish as a main component thereof is used, which varnish being formed by dissolving a petroleum resin or a polycarboxylic acid resin in an organic solvent. When this etching resist is printed in a prescribed pattern on a substrate and then thermally dried, it forms a coating film which is capable of resisting the etching agents. The pattern printing, however, has the problem that it is incapable of forming a fine circuit the desirability of which has been finding growing recognition in recent years.
The etching resists of the developing type are classified under the dry film type and the liquid type. Each of these types is further classified by the kind of a peeling liquid or remover under the solvent peeling type and the alkali peeling type. However, the restriction imposed on the use of the solvent peeling type has been gaining in severity on account of the problem of environmental pollution.
As a photosensitive dry film of the alkali peeling type, the film of the composition which, as disclosed in Japanese Patent Publication No. 3-68,376, has an ethylenically unsaturated compound, a photopolymerization initiator, and a film-forming polymer as main components thereof has been known. Since this film, for the sake of maintaining due strength, requires to possess a thickness of not less than 25-50 xcexcm, it exhibits the maximum attainable resolution of about 30 xcexcm at most and, consequently, entails the problem of being unable to form a fine pattern with resolution higher than the level just mentioned.
As a photosensitive liquid resist of the alkali peeling type, the composition which comprises the varnish having a polycarboxylic acid resin dissolved in an organic solvent, an ethylenically unsaturated compound, and a photopolymerization initiator has been known. The liquid resist formed by applying the composition to a substrate is enabled to acquire such resolution as approximating closely to 10 xcexcm by controlling the thickness of the applied layer of the composition. Since this composition requires liquefaction with an organic solvent, however, it has the problem of entailing environmental pollution.
The photosensitive resin compositions mentioned above are the negative type. When such a composition is adopted in the production of a multilayer printed circuit board, therefore, it inevitably poses the problem that the layer of the composition deposited on a substrate must be etched after the through holes formed in the substrate have been protected with a hole filling ink, for example. Specifically, the through holes in the substrate must be blocked up in advance with the hole filling ink because part of the layer of the photosensitive resin composition deposited on the substrate which has entered the through holes escapes being hardened even when the layer is exposed to light and the unhardened part of the composition, on being exposed to the action of development, flows out of the through holes. The hole filling ink which is a substance of the thermally drying type, however, has the problem that the thermal drying thereof consumes much time and the dried coating film of the ink fails to acquire fully satisfactory smoothness. In the field of the metalworking technology, the problem of inferior dimensional accuracy arises because the negative film of the composition to be used has a remarkedly different thermal expansion coefficient from metal.
The adoption of a positive type photoresist, therefore, has been gaining in popularity in the formation of a circuit pattern on a printed circuit board or the fabrication of a shadow mask which demands accuracy.
In the positive type etching resists being developed at present, naphthoquinone diazide compounds are usually adopted as photosensitive substances. Since the naphthoquinone diazide compounds are expensive, their adoption is problematic in terms of cost. In the formation of a very fine pattern, they possibly offer no fully satisfactory resolving power.
Published Japanese Patent Application, KOKAI (Early Publication) No. (hereinafter referred to briefly as xe2x80x9cJP-A-xe2x80x9d) 6-295,064 discloses the photosensitive composition which comprises a polymer containing a carboxyl group (polycarboxylic acid resin), a compound containing two or more vinyl ether groups in the molecule thereof, and a compound capable of generating an acid on exposure to an active energy ray. The coating film formed of this composition, when heated (primary heating), is insolubilized in a solvent or an aqueous alkali solution because the polycarboxylic acid resin is cross-linked by the addition reaction of the carboxyl group with the vinyl ether group. The insolubilized coating film, when further exposed to an active energy ray and then heated (secondary heating), the exposed part is again solubilized in the solvent or the aqueous alkali solution because the cross-linked structure is severed by the catalytic action of the acid generated in consequence of the exposure to the active energy ray. The resist pattern aimed at is formed by removing the exposed part by the developing treatment.
It may well be concluded that the composition which contains in combination the polycarboxylic acid resin, the multifunctional vinyl ether compound, and the compound capable of generating an acid on exposure to the active energy ray as mentioned above can basically answer the recent demand for the formation of very fine circuit patterns. Since this composition contains the multifunctional vinyl ether compound, however, it has the problem that the resist coating suffers the sensitivity and the resolving power thereof to be altered by the heating conditions and the like. Specifically, since the multifunctional vinyl ether compound which functions as a cross-linking agent for the polycarboxylic acid resin in the composition has high volatility, it is liable to volatilize during the course of the primary heating and change its content in the coating film. As a result, the composition has the problem that it fails to retain the sensitivity and the resolving power constant because the conversion of the carboxyl group and the vinyl ether group (the density of cross-linkage of the polycarboxylic acid resin) is varied by the conditions of the heating temperature, etc. Further, since the reaction of the carboxyl group with the multifunctional vinyl ether compound proceeds even at normal room temperature, the composition cannot be easily used unless it is prepared in the form of a two-component type composition. Thus, the composition is unfit for an etching resist. Further, the application of the composition of this nature to a substance poses the problem of polluting the environment because it requires use of an organic solvent.
Further, the recent rapid advance of semiconductor parts has been giving an impetus to the trend of electronic devices toward reduction in size and weight, exaltation of performance, and diversification of function and has been consequently directing printed circuit boards toward accelerated densification of patterned circuits. The demand for multilayer printed circuit boards has come to grow in concert with that for such high-density printed circuit boards. In the light of this trend, newer methods for the production of multilayer printed circuit boards have come to be proposed.
The conventional multilayer printed circuit board has been manufactured by a procedure which comprises superposing on the opposite surfaces of an inner-layer sheet having circuits formed thereon semi-cured prepreg sheets obtained in advance by impregnating such a substrate as glass cloth with a resin varnish and drying the wet substrate, further superposing thereon sequentially copper foils or copper-clad laminates as outer layers, for example, in the order mentioned, and simultaneously applying heat and pressure to the superposed layers by means of a multilayer-forming press in order to harden them, thereby completing a multilayer printed circuit board. Since this procedure requires the via holes intended to establish electric continuity between the component layers to be opened one by one with a drill or laser beam, it inevitably poses the problem of consuming an unduly long time and imposes a limit on the degree of densification of circuit patterns on account of the thickness of the prepreg and the diameter of the holes to be opened with a drill.
With a view to coping with this problem, the multilayer forming technology (build-up process) using a negative type liquid solder resist composition in the formation of an intermediate insulating layer has been proposed. Since this negative composition is a photo-setting type, the coating of the composition formed in a thickness necessary for the intermediate insulating layer cannot be fully photo-cured by one round of exposure to light. The technology, therefore, poses the problem of requiring a process of the steps of applying the composition, drying the applied layer of the composition, exposing the dried layer to light, and developing the exposed layer to be performed up to several repetitions in order for the applied layer to acquire a thickness necessary for the intermediate insulating layer. In this case, there are times when the additives such as a leveling agent contained in the resist induce the problem of ply separation. It also entails the problem that the organic solvent which has been used for liquefying the composition will be vaporized and diffused during the step of drying and consequently suffered to pollute the environment. The expulsion of the organic solvent results in inducing the dried applied layer to undergo volume shrinkage and rendering control of the thickness of the insulating layer of resin difficult.
In the circumstances, the process of thermally contact bonding a substrate containing a formed circuit and a copper foil through the medium of a layer of an alkali-soluble thermosetting resin composition, etching the surface copper foil exclusively in the parts thereof destined to be via holes, and thereafter dissolving the parts of the resin layer underlying the etched parts with an aqueous alkali solution thereby completing the via holes has been proposed (JP-A-5xe2x80x94218,651, JP-A-5-259,649, JP-A-5-343-850, and JP-A-7-170,073). The resin composition used in this process, however, produces a tack-free coating film by a procedure which comprises liquefying the alkali-soluble solid resin by dissolving the resin in an organic solvent, applying the liquefied resin to a copper foil or a printed circuit board containing a formed circuit, and thermally drying the applied layer of the resin. In this case, when the via holes are formed after the copper foil has been laminated and the coating film is thermally cured, there arises the problem that the organic solvent persisting in the resin composition vaporizes, generates bubbles, and causes separation between the copper foil and the layer of the resin composition. Further, the problem that the fine via holes cannot be formed with high accuracy ensues because the aqueous alkali solution does not exclusively dissolve the via hole parts and the aqueous alkali solution permeating through the via hole parts proceeds to dissolve the resin composition in the parts (peripheral walls) surrounding the via holes and impart a state of undercut to the resin composition.
A primary object of the present invention, therefore, is to provide a photosensitive resin composition which contains no organic solvent and, therefore, avoids inducing the otherwise possible evaporation of an organic solvent to cause environmental pollution or volume shrinkage of a dried coating film, and proves to be useful for the formation of a varying resin adhesive layer, the formation of the coating film of such a resist as etching resist or solder resist in a printed circuit board, and particularly the formation of a resin insulating layer containing via holes in the production of a multilayer printed circuit board.
A more specific object of the present invention is to provide a positive type photosensitive resin composition which, in the production of a multilayer printed circuit board by the copper foil laminating method, avoids posing the problem of suffering a residual organic solvent to cause separation of the copper foil during the course of thermal curing and, further owing to the positive quality of the composition, permits a tack-free coating film to be formed during the deposition of an insulating layer of a large thickness by collectively applying the composition in the form of a layer of a large thickness and then exposing the whole surface of the applied layer to light without requiring repetition of a procedure comprising the steps of application, drying, exposure to light, and development, and, as occasion demands, allows formation of via holes in the coating film of a large thickness by merely performing selective exposure to light, thermal treatment, and development exclusively in the parts assigned for the formation of via holes, and therefore proves to be usable as an intermediate insulating layer which is beneficial for the sake of forming stably shaped via holes without inducing ply separation.
A further object of the present invention is to provide a method which, in the field of production of a printed circuit board and metalworking, allows a positive resist pattern of high resolution to be formed stably at a low cost with high productivity.
Yet another object of the present invention is to provide a method which, in the impartation of a pattern to an intermediate insulating layer in a multilayer printed circuit board, permits formation of a tack-free coating film without entailing volume shrinkage by the irradiation with an active energy ray, i.e. a process having no possibility of diffusing a harmful organic solvent, and thereafter allows a resin insulating pattern excelling in such special qualities as heat resistance, electric insulation, and fastness of adhesion and possessing high resolution to be formed stably with satisfactory productivity by the selective exposure to an active energy ray having a shorter wavelength than the active energy ray mentioned above, heat treatment, development, and thermal curing.
To accomplish the objects mentioned above, the present invention provides a photosensitive resin composition characterized by comprising (A) a resinous compound containing an acid-decomposing ester group, (B) a compound containing one ethylenically unsaturated bond in its molecule and possessing a group capable of forming a carboxylic acid via decomposition by the action of an acid, (C) a compound capable of generating an acid on exposure to an active energy ray or actinic radiation (hereinafter referred to as xe2x80x9cphoto-acid generatorxe2x80x9d), and (D) a photo-radical polymerization initiator. Preferably the resinous compound (A) has a weight average molecular weight in the range of 500 to 40,000. Another preferable resinous compound has a weight average molecular weight in the range of 1,000 to 50,000.
In accordance with another aspect of the present invention, there is provided a photosensitive, thermosetting resin composition characterized by comprising (A) a resinous compound containing an acid-decomposing ester group and possessing a weight average molecular weight in the range of 500 to 40,000, (B) a compound containing one ethylenically unsaturated bond in its molecule and possessing a group capable of forming a carboxylic acid via decomposition by the action of an acid, (C) a photo-acid generator, (D) a photo-radical polymerization initiator, and (E) an epoxy resin.
The present invention further provides a method for forming a resist pattern or resin insulating pattern without involving a step for causing diffusion of a harmful organic solvent. This method is characterized by comprising a step of applying the photosensitive resin composition on a substrate, a primary exposure step of irradiating the coating film of the photosensitive resin composition with an active energy ray of a wavelength enough to activate the photo-radical polymerization initiator (D) and not enough to activate the photo-acid generator (C) thereby radically polymerizing the compound containing one ethylenically unsaturated bond in its molecule and possessing a group capable of forming a carboxylic acid via decomposition by the action of an acid (B) and consequently forming a tack-free coating film, a selective secondary exposure step of selectively irradiating the tack-free coating film with an active energy ray of a wavelength enough to activate the photo-acid generator (C) thereby decomposing the photo-acid generator (C) present in the irradiated part and forming an acid, a heating step of heating the coating film resulting from the selective secondary exposure step thereby decomposing the polymer containing an acid-decomposing ester group (A) and the radical polymer of the compound (B) mentioned above which are present in the part subjected to the selective secondary exposure and consequently rendering the coating film in that part soluble in an aqueous alkaline solution, and a step of developing the irradiated coating film with an aqueous alkaline solution thereby removing the part of the coating film subjected to the selective secondary exposure. Further, the present invention provides a method for forming a patterned resist or resin insulating layer excellent in heat resistance and insulating properties by thermally curing the resist pattern or resin insulating pattern formed as described above.